Electrospun doped nanofibers and process of preparation thereof

ABSTRACT

This invention is directed to nanofibers doped with an alkali salt having improved tensile properties, and process of preparation thereof.

FIELD OF THE INVENTION

This invention is directed to nanofibers doped with an alkali salt having improved tensile properties, and process of preparation thereof.

BACKGROUND OF THE INVENTION

Nanofibers are known for their wide range of applications in the fields of tissue engineering, textiles, filter material sensors and bioengineering applications, such as tissue regeneration, biosensors, recognition and filtration of viruses and drug molecules.

Well-established techniques for producing fibers for textiles having diameters in the micron length scales and with lengths in excess of meters are well known, including spun-bonding, meltblowing, dryspinning, conjugate spinning and CO₂ laser thinning produce fibers. Still, aligning and producing long fibers in the sub-micron and nanoscale diameters is challenging.

Electrospining (ES) is an efficient technique to produce nanometer size fibers for applications in textile, filtration, sensing, drug delivery and tissue engineering. The ES technique involves confining a polymer solution or melt to a syringe, with a metallic nozzle, wherein the metallic nozzle is connected to a high voltage. An electric field is established between the nozzle and a grounded metallic collector, the intensity is determined by the applied voltage and the distance between the nozzle and the collector. When the electric field exceeds the surface tension of the polymer solution (or melt), a polymer jet is formed from the polymer pendant drop. The jet is highly stretched while flying in the electric field and (in the case of a polymer solution) the solvent quickly evaporates, leading to the formation of a non-woven fibrous mat on the grounded collector. A number of key parameters, including viscosity, voltage, distance between the nozzle and the collector, and the conductivity of the polymer solution or melt, can be adjusted to control the diameter of electrospinned fibers.

An advantageous feature of electrospinning for practical use is that it easily allows microfibers to form a composite with a nonwoven fabric substrate. As mentioned above, electrospinning yields microfibers by applying a high voltage between the spinning nozzles and the counter electrode. If a nonwoven fabric substrate is placed between the spinning nozzles and the counter electrode, microfibers can be deposited on the substrate surface, thereby a composite fiber agglomerate can be readily prepared. This method can be applied to form a composite comprising polymers having different properties.

The finest fibers reported in the literature have diameters of 1.6 nm (Nylon-4,6) [C. B. Huang, S. L. Chen, C. L. Lai, D. H. Reneker, H. Qiu, Y. Ye, and H. Q. Hou, Nanotechnology 17, 1558-1563 (2006)]; and 5 nm (PAN/PEO)[Y. X. Zhou, M. Freitag, J. Hone, C. Staii, A. T. Johnson, N. J. Pinto, and A. G. MacDiamid, Appl. Phys. Lett. 83, 3800-3802 (2003)].

ES nanofibers are known to exhibit lower strength and stiffness, and larger variability, compared to their macroscopic (bulk) counterparts.

Thus, there is still a need to explore for methods and compositions to manipulate fiber properties in order to produce nanofiber materials having favorable properties for different applications.

SUMMARY OF THE INVENTION

In one embodiment, this invention is directed to a nanofiber comprising a polymer doped with an alkali salt; wherein said nanofiber has improved mechanical properties. In another embodiment the alkali salt is sodium chloride. In another embodiment the polymer is polymethyl methacrylate (PMMA), polyvinyl alcohol, polyethylene oxide, polyurethane, a polyamide, poly(vinyl chloride) or any combination thereof. In another embodiment, the polymer is polymethyl methacrylate (PMMA).

In one embodiment, this invention is directed to a process for the preparation of an electrospun nanofiber having improved mechanical properties comprising: (a) preparing a solution or a melt of an alkali salt and a polymer; (b) adding said solution or melt to an electrospinning system; (c) applying an electric field to yield liquid jets which are collected as fibers by a grounded collecting plate.

In one embodiment, this invention is directed to a rope having improved mechanical properties comprising a nanofiber of this invention, wherein said mechanical properties are stiffness, strength, toughness or any combination thereof.

In one embodiment, this invention is directed to a cable having improved mechanical properties comprising a nanofiber of this invention, wherein said improved mechanical properties are stiffness, strength, toughness or any combination thereof.

In one embodiment, this invention is directed to a nonwoven cloth having improved mechanical properties comprising a nanofiber of this invention; wherein said improved mechanical properties are stiffness, strength, toughness or any combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

FIG. 1: Transmission electron microscopy (TEM) images of PMMA (a and b) and PMMA-NaCl (c and d) electrospun fibers. The scale bars in (a) and (c) are 1 μm, and in (b) and (d) are 200 nm.

FIG. 2: Environmental scanning electron microscopy (ESEM) images of PMMA (a) and PMMA-NaCl (b) electrospun ropes. Scale bar is 20 μm.

FIG. 3: Diameter effects on tensile strength (a), Young's modulus (b), and toughness (c) of PMMA and PMMA-NaCl electrospun fibers.

It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

In one embodiment, this invention is directed to improved mechanical properties of nanofiber and process of preparation thereof, comprising a polymer doped with an alkali salt. In another embodiment, the salt is NaCl and the polymer is PMMA.

In one embodiment, this invention is directed to improved mechanical properties of a film and process of preparation thereof, comprising a polymer doped with an alkali salt. In another embodiment, the salt is NaCl and the polymer is PMMA.

In one embodiment, this invention is directed to an electrospun (ES) nanofiber and process of preparation thereof comprising a polymer doped with an alkali salt; wherein said nanofiber has improved mechanical properties. In another embodiment, the salt is NaCl and the polymer is PMMA.

In one embodiment, the nanofiber or film of this invention comprises a polymer. In one embodiment a “polymer” is refereed as a compound formed by the covalent joining of monomers or residues, when incorporated into a polymer. “Polymers” include copolymers, which are polymers comprising two or more different residues, such as block-copolymers. Polymers may have any topology, including, without limitation, straight-chain, branched-chain, star, dendritic, etc.

In another embodiment, the polymer may be crosslinked between one or more different polymers using appropriate crosslinking mechanisms. Crosslinking technology is a well known and understood phenomenon in which a crosslinking reagent reacts and forms covalent bonds between polymer chains to substantially improve molecular weight, chemical resistance, overall strength and resistance to mechanical degradation.

In another embodiment, non-limiting examples of useful polymers in the methods and structures described herein includes: polystyrene (PS), polyester, polyurethane, polyacrylamide, poly (methyl methacrylate) (PMMA), poly(2-hydroxyethylmethacrylate) (polyHEMA), polylactic acid (PLA), poly(glycolic acid) (PGA), poly(lactic-co-glycolic acid) (PLGA), poly(caprolactone), etc. In another embodiment, non-limiting list of useful biological polymers include fibrinogen, hyaluronic acid, collagen, gelatin, elastin, and polysaccharides, such as cellulose, amylose, dextran, chitin, chitosan, glycosaminoglycans.

In another embodiment, the polymer used for the formation of an ES nanofiber, rope or film is polymethyl methacrylate (PMMA), polyvinyl alcohol, polyethylene oxide, polyurethane, a polyamide, poly(vinyl chloride) or any combination thereof. In another embodiment, the polymer is a copolymer. In another embodiment, the polymer is polymethyl methacrylate (PMMA). In another embodiment the polymer is polyvinyl alcohol. In another embodiment, the polymer is polyethylene oxide. In another embodiment, the polymer is polyurethane. In another embodiment the polymer is a polyamide. In another embodiment the polymer is poly(vinyl chloride).

In one embodiment, the molecular weight of the polymer of this invention ranges between about 20,000-500,000 g/mol. In another embodiment, the molecular weight of the polymer of this invention ranges between about 100,000-300,000 g/mol.

In one embodiment, the nanofiber or film of this invention is doped with an alkali salt. An “alkali salt” is referred to an ionic salt of alkali metal such as sodium, potassium, lithium or cesium. In another embodiment, the alkali salt is NaCl. In another embodiment, the alkali salt is KCl. In another embodiment, the alkali salt is LiCl. In another embodiment, the alkali salt is sodium carbonate. In another embodiment, the alkali salt is sodium hydroxide. In another embodiment, the alkali salt is NaBr. In another embodiment, the alkali salt is NO. In another embodiment, the alkali salt is CsCl. In another embodiment, an alkali salt is a combination of two or more alkali salts as described hereinabove.

In one embodiment, this invention is directed to an ES nanofiber and process of preparation thereof, wherein said nanofiber comprises polymethyl methacrylate (PMMA) doped with NaCl.

In one embodiment, the nanofiber of this invention is doped with an alkali salt. In another embodiment, the concentration of the salt is between about 0.2-0.4 weight percentage from said polymer. In another embodiment, the concentration of the salt is between about 0.2-0.5 weight percentage from said polymer. In another embodiment, the concentration of the salt is between about 0.25-0.3 weight percentage from said polymer. In another embodiment, the concentration of the salt is about 0.25 weight percentage from said polymer. In another embodiment, the molar ratio between the alkali salt and the polymer is between about 10:1 to 20:1, thus between 10-20 molecules of salt are associate with every polymer chain. In another embodiment, the molar ratio between the alkali salt and the polymer is between about 10:1 to 15:1. In another embodiment, the molar ratio between the alkali salt and the polymer is between about 15:1 to 20:1. In another embodiment, the molar ratio between the alkali salt and the polymer is about 15:1.

In one embodiment, this invention is directed to improved mechanical properties of a nanofiber or film of this invention. In another embodiment, improved mechanical properties are obtained by doping the polymer used for the preparation of the nanofiber or film with an alakali salt. In another embodiment, the mechanical properties include improved tensile properties, improved toughness, improved strength, improved stiffness or any combination thereof.

In one embodiment, the term “strength” refers to tensile strength. A polymer has high tensile strength if it is strong when one pulls by applying pressure on it. The stress/force needed to break the sample by increasing the amount of force, and stress on the sample is the tensile strength of the material. Stress and strength are measured in units of force divided by units of area, usually N/m² also called Pascal. Stress and strength can also be measured in megapascals (MPa) or gigapascals (GPa).

In one embodiment, the nanofiber of this invention provides increase in tensile strength by between 75%-150% compared to undoped nanofiber. In another embodiment, the nanofiber of this invention provides increase in tensile strength by between 80%-100% compared to undoped nanofiber. In another embodiment, the nanofiber of this invention provides increase in tensile strength by between 75%-100% compared to undoped nanofiber. In another embodiment, the nanofiber of this invention provides increase in tensile strength by between 75%-85% compared to undoped nanofiber. In another embodiment, the nanofiber of this invention provides increase in tensile strength as described in Table 1 and Example 4(b).

In one embodiment, the film of this invention provides an increase in tensile strength of between 10% to 50% compared to an undoped film. In another embodiment, the film of this invention provides an increase in tensile strength of between 10% to 30% compared to an undoped film. In another embodiment, the film of this invention provides an increase in tensile strength of between 10% to 20% compared to an undoped film. In another embodiment, the film of this invention provides an increase in tensile strength as described in Table 1 and Example 4(b).

When applying tensile stress, the sample deforms by stretching, and thereby elongating (becoming longer). The percent of elongation is the length change of the polymer sample during stretching (L−L₀), divided by the original length of the sample (L₀), and then multiplied by 100. In one embodiment, the nanofiber, rope or film of this invention comprising a polymer doped with alkali salt provides enhanced molecular alignment. In another embodiment, ES nanofiber of this invention doped with alkali salt provides enhanced molecular alignment. In another embodiment, the salt is NaCl and the polymer is PMMA.

The term “tensile modulus” is referred herein as “The Young Modulus”, E which is one of the properties of the polymer that describes its stiffness. The term “stiffness” is referred herein to “tensile modulus”. Tensile modulus is measured by measuring the stress on the material, and the elongation the sample undergoes at low stress level. One can determine the Young Modulus from the initial slope (the linear part) of the stress-strain curve. High tensile modulus means the polymer resists deformation. Modulus is measured by calculating stress and dividing by elongation. Modulus is expressed in the same units as strength, such as N/m² or Pascal. Fibers with larger diameters will be stiffer in bending.

In another embodiment, the nanofiber of this invention provides an increase in tensile modulus of between 100%-200% compared to an undoped nanofiber. In another embodiment, the nanofiber of this invention provides an increase in tensile modulus of between 100%-150% compared to an undoped nanofiber. In another embodiment, the nanofiber of this invention provides increase in tensile modulus of between 100%-130% compared to an undoped nanofiber. In another embodiment, the nanofiber of this invention provides an increase in tensile modulus as described in Table 1 and Example 4(b).

In one embodiment, the film of this invention provides an increase in tensile modulus of between 20% to 50% compared to an undoped film. In another embodiment, the film of this invention provides an increase in tensile modulus of between 20% to 30% compared to an undoped film. In another embodiment, the film of this invention provides increase in tensile modulus of between 25% to 40% compared to undoped film. In another embodiment, the film of this invention provides increase in tensile modulus as described in Table 1 and Example 4(b).

The term “toughness” as described herein is the energy a sample can absorb before it breaks. In another embodiment, the nanofibers of this invention provide increase in toughness by between 100% to 400% compared to undoped nanofiber. In another embodiment, the nanofibers of this invention provide increase in toughness by between 100% to 300% compared to undoped nanofiber. In another embodiment, the nanofibers of this invention provide increase in toughness by between 100% to 200% compared to an undoped nanofiber. In another embodiment, the nanofiber of this invention provides an increase in toughness as described in Table 1 and Example 4(b).

In one embodiment, the Young's modulus and the strength of nanofibers, doped and undoped, strongly increase with a decrease in diameter, and (mainly under 500 nm) so does toughness, as seen in FIG. 3. In one embodiment, polymer fiber doped with NaCl clearly show higher mechanical properties at all diameters (and especially under 500 nm) than the pure polymer fibers in the same diameter ranges.

In one embodiment, the average diameter of the nanofiber of this invention is between 200 nm to about 1 μm. In another embodiment, the average diameter of the nanofiber of this invention is between 300-600 nm. In another embodiment, the average diameter of the nanofibers of this invention is between 400-800 nm.

In one embodiment, this invention is directed to ES nanofibers and process of preparation thereof, wherein the nanofibers are doped with an alkali salt. In another embodiment the fibers morphology show that the surface of electrospun fibers are smooth, and the diameter is uniform all along, with or without salt doping—specifically, the elimination of unwanted beads and the production of thinner fibers. In another embodiment, FIG. 1 provides TEM images of electrospun fibers with or without NaCl wherein doping with NaCl had no visible effect on the fiber morphology and no beads were produced. In one embodiment such doping has the effect to alter the net charge density of the polymer solution, thereby eliminating beads and generating thinner fibers.

In one embodiment, this invention is directed to a process for the preparation of an electrospun (ES) nanofiber having improved mechanical properties comprising: (a) preparing a solution or a melt of an alkali salt and a polymer; (b) adding said solution or melt to an electrospinning system; (c) applying an electric field to yield liquid jets which are collected as fibers by a grounded collecting plate. In another embodiment, the collected fibers are further washed. In another embodiment, the collected fibers are washed with an organic solvent. In another embodiment, the collected fibers are washed with ethanol or methanol.

In another embodiment, electrospinning (ES) is a known technique to obtain nanofibers. Electro spinning, uses high voltage to a capillary (nozzle) filled with the polymer fluid to be spun to obtain the fiber from a liquid (or melt) polymer.

In one embodiment, the process of this invention includes a solution of an alkali salt and a polymer. In another embodiment, the solution includes a polar organic solvent. In another embodiment, the solution includes DMF, DMSO, THF, DMA, or any combination thereof as solvents. In another embodiment the solvent is DMF.

A jet of charged fluid polymer sprays out the bottom of a nozzle, an electric field forces the stream to whip back and forth, stretching the fiber lengthwise so its diameter is reduced. The fiber forms a thin deposit as it hits the grounded collector below the nozzle. These electrospun deposits have a unique combination of stretchiness and strength, and are easy to handle, making them suitable for a wide range of applications. Unlike conventional fiber spinning techniques (wet spinning, dry spinning, melt spinning, gel spinning), which are capable of producing polymer fibers with diameters down to the micrometer range, electrospinning is a process capable of producing polymer fibers in the nanometer diameter range. Electrospinning is an efficient fabrication process that can be utilized to assemble fibrous polymer composed of fiber diameters ranging from several microns down to fibers with diameter lower than 100 nm. This electrostatic processing method uses a high-voltage electric field to form solid fibers from a polymeric fluid stream (solution or melt) delivered through a millimeter-scale nozzle.

The following parameters and processing variables affect the electrospinning process: (i) molecular weight, molecular-weight distribution and architecture (branched, linear etc.) of the polymer and solution properties (viscosity, conductivity and surface tension); (ii) Process parameters such as electric potential, flow rate and concentration, distance between the nozzle and collection plate; and (iii) ambient parameters (temperature, humidity and air velocity in the chamber).

In one embodiment, this invention is directed to process for the preparation of improved mechanical properties of a nanofiber comprising the steps of adding a solution or melt to an electrospinning system and applying an electric field to yield liquid jets. In another embodiment, the electric field is between 50-100 kV/m. In another embodiment, the electric field is between 60-80 kV/m. In another embodiment, the electric field is between 60-70 kV/m. In another embodiment, the electric field is between 65-70 kV/m. In another embodiment, the electric field is about 66 kV/m.

In one embodiment the concentration of the polymer used in the electrospinning system for the formation of an ES nanofiber is between 50-100 mg/mL. In another embodiment the concentration of the polymer used in the electrospinning system for the formation of an ES nanofiber is between 60-90 mg/mL. In another embodiment the concentration of the polymer used in the electrospinning system for the formation of an ES nanofiber is between 70-90 mg/mL. In another embodiment the concentration of the polymer used in the electrospinning system for the formation of an ES nanofiber is about 80 mg/mL.

In one embodiment, this invention is directed to a rope having improved mechanical properties comprising the nanofiber of the invention; wherein said mechanical properties are stiffness, strength, toughness or any combination thereof. In another embodiment, the rope of this invention is prepared by using the process of this invention for the preparation of a nanofiber following by simultaneously alignment and twisting of the nanofiber on the collecting electrode which is connected to a rotator motor to form a rope.

In one embodiment, the rope of this invention provides an increase in tensile modulus of between 100 to 200% compared to an undoped rope. In another embodiment, the rope of this invention provides an increase in tensile modulus of between 100% to 150% compared to an undoped rope. In another embodiment, the rope of this invention provides an increase in tensile modulus as described in Table 1 and Example 4(b).

In one embodiment, the rope of this invention provides an increase in toughness of about 15 to 30 times more compared to an undoped rope. In another embodiment, the rope of this invention provides increase in toughness of about 15 to 20 times more compared to undoped rope. In another embodiment, the rope of this invention provides an increase in toughness as described in Table 1 and Example 4(b).

In one embodiment, this invention is directed to a film having improved mechanical properties comprising a polymer doped with an alkali salt; wherein said mechanical properties are stiffness, strength, toughness or any combination thereof. In another embodiment, the effect of the alkali salt doping is smaller in films compared to fibers and/or ropes. In another embodiment, the film of this invention is prepared by any known technique known in the art. Non limiting examples of methods of preparing films include drop casting, vapor phase deposition, spin coating, electrochemical deposition, precipitation, crystallization, chemical deposition or Langmuir blodgett deposition.

In one embodiment, multiple fibers of this invention may be combined to form textiles, metal wires, cables, polymeric cables, optic cables, ropes, etc. Multiple fibers may also be used in fiber reinforced composites, wherein multiple fibers/textile layers are embedded in another material.

In one embodiment, this invention is directed to a cable having improved mechanical properties comprising the nanofiber of this invention, wherein said improved mechanical properties are stiffness, strength, toughness or any combination thereof.

In another embodiment, the nanofiber of the present invention is useful as a reinforcing material for an optical cable in an optical communication field.

In one embodiment, this invention is directed to a nonwoven cloth having improved mechanical properties comprising the nanofiber of this invention; wherein said improved mechanical properties are stiffness, strength, toughness or any combination thereof.

As used herein, “nano” refers to sizes of between about 1-1000 nanometers (nm) (e.g., 1-100 nm).

The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise

The term “about” or “approximately” as used herein means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, “about” can mean within 1 or more than 1 standard deviations, per the practice in the art. Alternatively, “about” can mean a range of up to 20%, and preferably up to 10% of a given value; such as within 7.5%, within 5%, within 2%, within 1%, within 0.5% of a given value.

In one embodiment, the nanofiber, rope, cable, nonwoven cloth or film of this invention may be characterized using methods known in the art. For example, scanning electron microscope (SEM) and Transmission electron microscopy (TEM) may provide evidence of the fiber surface morphology and diameter. Raman spectroscopy may provide information on the intermolecular interactions between nanoparticles and the fibers. Other methods include atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), tensile testing and x-ray diffraction.

Other characterization methods are available. For example, thermal properties may be investigated using Thermogravimetric Analysis (TGA) or Differential Scanning calorimetry (DSC). TGA may provide information on the decomposition steps and decomposition temperatures of the materials. To complement this information, DSC may provide information on phase transitions, such as glass transition temperatures (Tg), crystallization and melting temperatures.

Mechanical characterization of the fibers may be performed using a microtensile tester. This technique provides information such as Young modulus, stiffness, tensile strength and tensile load.

The following examples are presented in order to more fully illustrate the preferred embodiments of the invention. They should in no way, however, be construed as limiting the broad scope of the invention.

EXAMPLES Example 1 Preparation of ES Nanofibers

a. Preparation of the Solution

A solution consisting of 2 mg NaCl powder (Sodium Chloride GR, MerckKGaA) added to 10 ml dimethylformamide (DMF, Baker) was prepared by sonicating for half an hour to get a clear mixture. This was followed by the addition of 800 mg PMMA (Aldrich), stirred overnight at 70° C., resulting in an 80 mg/ml PMMA solution, with a 0.25% by weight NaCl concentration.

b. Fiber Spinning

ES nanofibers were prepared using the solution described herein above (Example 1, item (a)) according to the procedure described in X. M. Sui and H. D. Wagner, Nano Lett. 9, 1423-1426 (2009); X. M. Sui, S. Giordani, M. Prato, and H. D. Wagner, Appl. Phys. Lett. 95, 233113 (2009); L. Q. Liu, D. Tasis, M. Prato, and H. D. Wagner, Adv. Mater. 19, 1228-1233 (2007); which are hereby incorporated by reference. Briefly, a high voltage (electrical field strength: 66 kV/m) was applied (Glassman High Voltage Inc.) through a steel clamp mount on a 27G stainless steel needle attached to a plastic syringe containing a polymer solution. The syringe was connected to a syringe pump (Longer Pump, TJ-2A) and infused at a flow rate of 300 μl/hour. Several parallel gaps were cut out in a stiff aluminum foil using a scalpel. The foil was grounded to function as a collector for the fibers. The parallel gaps in the conducting electrodes were used to generate aligned, parallel fibers. The fibers underwent a post-treatment into methanol for 8 hours, which is designed to extract any residual solvent. This was followed by overnight heating at 40° C. in a vacuum oven. All specimens were stored in a desiccator under light vacuum prior to mechanical testing.

Ropes made out of single fibers were prepared according to the procedure described in L. Q. Liu, M. Eder, I. Burgert, D. Tasis, M. Prato, and H. D. Wagner, Appl. Phys. Lett. 90, 083108 (2007); which is hereby incorporated by reference.

The ropes of this invention had a diameter of between 30-40-microns.

Example 2 Preparation of Films by Casting and Evaporation

PMMA films were also prepared for comparison, as bulk counterpart. A solution identical to the one used for the ES fibers (see Example 1, item (a)-hereinabove) was cast in an aluminum dish, then stored in a vacuum oven at 100° C. for 48 hours to let the DMF solvent evaporate. The resulting film was then cut into 11×3×0.07 mm³ rectangular stripes using parallel razor blades mounted on a micro-manipulator.

Example 3 Tensile Tests of ES-Fibers

Tensile testing of the ES fibers, the ropes, and the bulk films was carried out with a home-built nano-tensile tester, a home-built micro-tensile tester, and a Minimat-2000 (Rheometric Scientific), respectively. The fibers were cut at a spot close to the edge of the gap (above the foil) so that each of them could easily be picked up by means of a glue droplet previously deposited on the conical end of an AFM tip. Such pick up was performed by a nano-manipulator (Kleindiek Nanotechnik, NanoControl NC-2-3). The liquid epoxy glue solidified usually within 40 min, following which the nanofiber-tip complex could easily be removed from the Aluminum foil. Subsequently, the opposite (free) end of the nanofiber was plunged into a large drop of liquid epoxy glue spread on a stub attached to the manipulator. The epoxy drop solidified within 40 minutes, as before. A tensile test could then be performed by inducing tension in the fiber by pulling the ES fiber with the nano-manipulator. The nanofiber was stretched by moving the polymer droplet horizontally, forcing the cantilever to bend in the same direction. The total deformation is thus a combination of the fiber deformation and the cantilever deflection. At a critical value during the extension the nanofiber failed, generally near the middle of the fiber. The cantilever deflection was continuously recorded and converted into a force using the thermal-noise calibration method of Sader et al. as described in J. E. Sader, J. W. M. Chon, and P. Mulvaney, Rev. Sci. Instrum. 70, 3967-3969 (1999); which is hereby incorporated by reference. The tensile strength of the nanofiber was calculated by dividing the maximum bending force of the cantilever by the original cross sectional area of the nanofiber. The fiber elongation was measured using the distance between both fiber ends as the gauge length. The deformation speed was 80 μm/min. All experiments were video-recorded and subsequently analyzed. Further experimental details can be found in X. M. Sui and H. D. Wagner, Nano Lett. 9, 1423-1426 (2009). X. M. Sui, S. Giordani, M. Prato, and H. D. Wagner, Appl. Phys. Lett. 95, 233113 (2009); which are hereby incorporated by reference.

For the tensile test of PMMA films, a 200 N load cell on Minimat, at a crosshead speed of 0.8-1.0 mm/min was used.

Morphology

Segments of ES nanofibers as prepared hereinabove were examined by transmission electron microscope (TEM, Philips CM-120, acceleration voltage 120 kV). The fibers were deposited onto carbon-film-coated 300 mesh copper grids without any post treatment. Ropes were examined by environmental scanning electron microscope (ESEM FEI XL-30, working voltage is 5 kV). The ropes were placed on an environmental scanning electron microscope (ESEM) stub covered with carbon tape, and covered with a thin sputtered layer of gold-platinum.

Example 4 Mechanical Characterization of ES-Fibers, Ropes and Films, Prepared According to Example 1

a. Morphology

The transmission electron microscopy (TEM) images in FIG. 1 show that the surface of electrospun fibers is smooth, and the diameter is uniform all along, with or without NaCl doping. Thus, doping with NaCl had no visible effect on the fiber morphology, and no beads were observed. The minute NaCl crystals could not be observed, most likely because of their very low concentration. FIG. 2 are typical environmental scanning electron microscope (ESEM) views of PMMA (a) and PMMA-NaCl (b) electrospun ropes.

b. Mechanical Properties

The main point to be elucidated concerns the effect of doping with minute amounts of NaCl on the mechanical properties of ES-polymethylmethacrylate (PMMA) fibers, ropes, and films. As seen in Table 1, which lists all tensile test results, the average strength and modulus of NaCl doped PMMA nanofibers increase by 79% and 127%, respectively. The only indicator of toughness that may be defined for thin ES fibers is their energy absorption under tensile deformation, or “tensile toughness” W=∫σdε (where σ is the tensile stress and ε the deformability), which is simply the energy per unit volume that the nanofiber material absorbs before rupturing (the area under the stress-strain curve). As seen in Table 1, NaCl doped PMMA nanofibers have about twice the toughness compared to pure PMMA fibers, due to their higher strength and strain to failure.

TABLE 1 Tensile test results. Values in parenthesis are standard deviations. Weibull distribution ε_(max) (%) σ_(f) (Mpa) E (Gpa) W (MJ/m³) α (MPa) β R² Fibers PMMA 37.0 (22.5) 98.7 (46.0) 1.1 (0.5) 32.1 (28.7) 111.3 2.50 0.92 PMMA-NaCl 48.2 (35.0) 176.8 (54.6)  2.5 (0.8) 79.7 (72.7) 195.5 3.85 0.94 p value of t-test* 0.35 6.72E−04 1.06E−04 0.051 Ropes PMMA 5.0 (2.0) 35.0 (9.8)  0.9 (0.2) 1.2 (0.8) 38.5 4.16 0.87 PMMA-NaCl 28.0 (11.0) 77.5 (11.7) 1.8 (0.3) 19.0 (9.1)  82.3 7.87 0.91 p value of t-test 1.07E−06 5.65E−11 8.97E−10 2.62E−06 Films PMMA 6.0 (1.1) 74.9 (7.9) 2.0 (0.1) 2.6 (0.8) 78.7 9.96 0.85 PMMA-NaCl 5.6 (1.0)  86.3 (13.3) 2.6 (0.5) 2.8 (0.9) 91.7 7.82 0.90 p value of t-test 0.31 7.56E−03 3.57E−04 0.495 *See text for details

These data can be explained by increase of the net charge density carried by the moving jet, and therefore increases the electric force that the polymer solution experienced. Given the concentration of NaCl in PMMA (0.25 wt. %), and based on the known molecular weights of both PMMA and NaCl, the molar ratio of NaCl to PMMA chain was calculated to be 15:1. In other words, an average of 15 NaCl molecules associates with every PMMA chain, produced a strong electric force on the polymer chain along the jet from the solution. The strength of this force, which was mainly shearing, resulted in a much better alignment of the polymer chain, compared to a solution without dopant, leading to higher stiffness and strength toughness. Doped fibers exhibited almost no necking compared to undoped ones. This confirms the better chain alignment of doped fibers at the formation stage, since necking was usually observed when chains were originally more random and when more entanglements were present, as indeed was the case in undoped PMMA fibers. A similar situation arised with crazing in case of bulk material.

To demonstrate that the reinforcement of ES fibers comes from the ionic salt assisted polymer alignment, a comparison with PMMA films with and without salt (Table 1) revealed that the strain of doped PMMA films was smaller compared to undoped ones, and that the strength and Young's modulus were improved by 15% and 30%, respectively. The effect of NaCl doping therefore appears to be much smaller in films and, unlike the fibers, it cannot be explained by alignment due to electrical forces. A possible explanation for the generally smaller improvement in mechanical properties of PMMA film is during film preparation the polymer solution spreads randomly and isotropically on the substrate against the surface tension, and no preferential molecular alignment is present in the films. Since solvent evaporation is relatively slow, the polymer chains are assumed to remain in their equilibrium configuration. This implies that the significant improvement of mechanical properties observed in ES fibers arises from the NaCl-assisted alignment of PMMA chains. As a matter of fact, this is observed for ropes as well, with the added advantage (compared to fibers) that ropes—a structural element that can more easily be handled-exhibit higher reliability (lower coefficients of variation).

A simple statistical analysis (t-test) was used to probe the effect of doping in fibers, ropes and films. In the fiber case the effect of diameter (see discussion below and FIG. 3) was ignored in the analysis, for simplicity. The specific null hypothesis was that there is no significant difference in the mechanical properties due to NaCl doping. Referring to Table 1, in the case of the ES fibers, the p values were smaller than the selected critical value (0.05, two tailed) for both strength and Young's modulus, therefore the null hypothesis was rejected, and it can be safely concluded that the effect of doping was indeed significant for these two properties. In the case of the films, the same conclusions were reached but the effect of doping was much larger in the fibers. For the ropes, the effect of doping is strongly significant for all mechanical properties.

The Young's modulus and the strength of PMMA fibers, doped and undoped, strongly increase with a decrease in diameter, and (mainly under 500 nm) so does toughness, as seen in FIG. 3. Such diameter effect was observed previously in our laboratory, and by Arinstein et al. with other ES polymer fibers. Moreover, PMMA fiber doped with NaCl clearly show higher mechanical properties at all diameters (and especially under 500 nm) than the pure PMMA fibers in the same diameter ranges.

In strong fibers such as carbon and glass, and more recently in PMMA ES fibers reinforced by carbon nanotubes, the severity of the flaws follows a Poisson distribution and the strength of a fiber is then determined by the most severe flaw. This behavior was conveniently modeled by a Weibull two-parameter distribution, and the same was done here with both types of ES nanofibers. If σ_(f) is the failure strength of the nanofiber, the cumulative distribution function F(σ_(f)) for the two-parameter Weibull distribution is given by

${F\left( \sigma_{f} \right)} = {1 - {\exp \left( {- \left( \frac{\sigma_{f}}{\alpha} \right)^{\beta}} \right)}}$

where F(σ_(f)) is the probability of failure, α is the scale parameter (with dimensions of stress), and β is the shape parameter (no dimensions). Most fibers used in composite materials (carbon/graphite, Kevlar, glass) do follow quite accurately the Weibull-Poisson statistical model, and this model is applied here for both fiber and film specimens. The scale and shape parameters, α and β, can easily be calculated from a probability plot based on the cumulative distribution function F(σ_(f)). These two parameters are indicators of the mean failure strength of the population, and of its variability. A higher β value indicates lower variability (thus, the material is more reliable). The Weibull model was found to be a good fit in all 4 cases reported in Table 1, as indicated by the R² value, despite the fact that specimens with different diameters were present. As seen in Table 1, the scale parameters are close to the average tensile strengths, and follow similar trends. An important conclusion based on the Weibull shape parameter is that its value for the fibers was increased through NaCl doping, from 2.5 to 3.9, an indication of lower strength data spread (or higher reliability) following doping. The same is true in the case of ropes. Note however that (i) the shape parameter of both types of ES fibers was nevertheless relatively low (similar to brittle fibers such as glass), and (ii) the shape parameter of the films was much higher (similar to tough polymer fibers such as Kevlar) but that doping leads to a slight decrease in this parameter. The shape parameter of ropes is an intermediate between fibers and films.

Alignment of the polymer chains in the direction of tension is improved by doping, a smaller number of configurational molecular defects (such as agglomerates, free volume etc) is present in the structure, thus the tensile strength is closer to the intrinsic strength of the polymer chain and becomes more predictable. In film specimens, on the other hand, NaCl doping does not result in Weibull scale or shape parameter improvement since the film preparation procedure does not yield any molecular alignment. An increase in strength and stiffness is observed upon doping of PMMA films, possibly because of the presence of tiny NaCl crystals playing the role of reinforcement. On the other hand, the observed slight decrease in the shape parameter observed upon NaCl doping of the films may or may not be significant, in view of the relatively large value (8 to 10) of this parameter (which reflects a much lower strength variability compared to fibers). As to ropes, their mechanical behavior (and variability in mechanical properties) obeys classical expectations from fiber bundle theory.

While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 

What is claimed:
 1. A nanofiber comprising a polymer doped with alkali salt, wherein said nanofiber has improved mechanical properties.
 2. The nanofiber of claim 1, wherein said polymer is polymethyl methacrylate (PMMA), polyvinyl alcohol, polyethylene oxide, polyurethane, a polyamide, poly(vinyl chloride) or any combination thereof.
 3. The nanofiber of claim 1, wherein said alkali salt is sodium chloride (NaCl).
 4. The nanofiber of claim 1, wherein said polymer is polymethyl methacrylate (PMMA) and said salt is NaCl.
 5. The nanofiber of claim 1, wherein said alkali salt is between about 0.2-0.4% weight percentage from said polymer.
 6. The nanofiber of claim 1, wherein said mechanical properties are stiffness, strength, toughness or any combination thereof.
 7. The nanofiber of claim 5, wherein said strength is improved by between about 75-100%.
 8. The nanofiber of claim 1, wherein the diameter of said nanofiber is between 200 nm to 1 micron
 9. A process for the preparation of an electrospun nanofiber having improved mechanical properties comprising: (a) preparing a solution or a melt of an alkali salt and a polymer; (b) adding said solution or melt to an electrospinning system; (c) applying an electric field to yield liquid jets which are collected as fibers by a grounded collecting plate.
 10. The process of claim 9, wherein said polymer is polymethyl methacrylate (PMMA), polyvinyl alcohol, polyethylene oxide, polyurethane, a polyamide, poly(vinyl chloride) or any combination thereof.
 11. The process of claim 9, wherein said alkali salt is sodium chloride (NaCl).
 12. The process of claim 9, wherein said polymer is polymethyl methacrylate (PMMA) and said salt is NaCl.
 13. The process of claim 9, wherein said alkali salt is in between about 0.2-0.4% weight percentage from said polymer.
 14. The process of claim 9, wherein said mechanical properties are stiffness, strength, toughness or combination thereof.
 15. The process of claim 14, wherein said strength is improved by between about 75-100%.
 16. The process of claim 9, wherein the diameter of said nanofiber is between 200 nm to 1 micron.
 17. The process of claim 9, wherein said electric field is between 50-100 kV/m.
 18. The process of claim 9, wherein said collected fibers were further treated with methanol.
 19. A rope having improved mechanical properties comprising said nanofiber of claim 1; wherein said mechanical properties are stiffness, strength, toughness or any combination thereof.
 20. A cable having improved mechanical properties comprising said nanofiber of claim 1, wherein said improved mechanical properties are stiffness, strength, toughness or any combination thereof. 